EFFECTS OF THE REFRACTIVE-INDEX SPECTRAL MODEL ON THE IRRADIANCE VARIANCE OF A GAUSSIAN-BEAM

The normalized irradiance variance of a Gaussian beam in the weak-fluc tuation regime is examined numerically with the use of various spectra l models for refractive index fluctuations. The Kolmogorov, Tatarskii, and modified von Karman spectral models are chosen as conventional mo dels, while the Hill numerical spectral model and the Andrews analytic approximation to the Hill model are selected to feature the character istic bump at high wave numbers. The latter two models are known to pr edict higher scintillation levels than conventional spectral models wh en the Fresnel zone and the inner scale are of comparable size. Outer scale effects appear minimal near the centerline of the beam but can r educe off-axis scintillation significantly. Inner scale effects are pr ominent on axis as well as off axis, although they sometimes tend to d iminish near the diffractive beam edge. Analytic approximations are de veloped for the irradiance variance based on the Kolmogorov, modified von Karman, and Andrews spectral models. These analytic expressions ar e generally in excellent agreement with numerical results.